1. Field of the Invention
The present invention relates to a method and an apparatus for injection molding a semi-molten metal into a mold cavity to produce a thick molded article
2. Prior Art
As a method of producing a metal molded article having better internal quality than that made by die-casting, a semi-molten metal injection molding method wherein a molten metal (magnesium alloy), in a semi-melting state at a temperature of not more than a liquidus temperature of the metal material, is injected into a cavity of a mold has conventionally been known, as disclosed in Japanese Patent Publication JP-B2-15620 (1990), corresponding to U.S. Pat. No. 4694882. Since the method for injection molding a semi-molten metal makes it possible to mold the metal at a relatively low temperature, the useful life of molds can be made longer than that of a mold used in die-casting, and moreover the high molding accuracy can be maintained for a long time of repeated moldings.
When molding a thick-wall metallic article having a thickness of not less than 5.0 mm in a product portion corresponding to a cavity by injection molding, a die casting method is apt to cause disturbance in the molten metal flow, during filling the cavity with the molten metal, which leads to gas entrapment and lower internal quality. Therefore, the injection molding method capable of injecting the semi-molten metal in a state of a laminar flow is more suitable than the die casting because of its high viscosity in the presence of a solid phase in the melt.
However, even if the semi-molten metal injection molding method is applied to the thick-wall article, filling and gas defects, shrinkage cavity, etc, cannot be avoided in the thick-wall parts of the molded article when setting the same molding conditions as in the case of producing a conventional relatively thin-wall molded article. Thus, for the injection molding process it is difficult to mold the thick molded article with high quality.